how to use the gbhe mixing guides

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Process Engineering Guide: GBHE-PEG-MIX-700

How to Use the GBHE Mixing Guides Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: How to Use the GBHE Mixing Guides

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 THE MIXING GUIDES 3 4.1 Mixing Guides 3 4.2 GBHE Mixing and Agitation Manual 3 5 DEVICE SELECTION 3 6 MIXING QUESTIONNAIRE 5

6.1 What is being mixed? 5 6.2 Why is it being mixed? 5 6.3 How is it to be mixed? 5 6.4 Is Heat Transfer Important? 6 6.5 Is Mixing Time Important? 6 6.6 Is Inventory Important? 6 6.7 Is Subsequent Phase Separation Important? 6 6.8 What Quantities? 6 6.9 What are the Selection Criteria? 6 6.10 What Data are required? 6



7 BASICS 7 7.1 Bulk Movement 8 7.2 Shear and Elongation 8 7.3 Turbulent Diffusion 8 7.4 Molecular Diffusion 9 7.5 Mixing Mechanisms 9 APPENDICES A ROTATING MIXING DEVICES 11 B MIXING DEVICES WITHOUT MOVING PARTS 18



TABLES 1 SUMMARY OF TYPICAL USES OF MIXING DEVICES 4 FIGURES 1 CHARACTERISTICS OF VARIOUS DEVICES IN TERMS

OF BULK FLOW 9 2 TANK LAYOUT 11 3 PITCHED BLADE TURBINE 12 4 DISC TURBINE 12 5 PROPELLER 13 6 RETREAT CURVED BLADE TURBINE 13 7 CONCAVE BLADE TURBINE 14 8 'GASFOIL' AGITATOR 14 9 'HYDROFOIL' AGITATOR 14 10 HELICAL SCREW STIRRER 15 11 HELICAL RIBBON STIRRER 15 12 ANCHOR AGITATOR 16 13 BENT ANCHOR 16 14 ”SAWTOOTH” DISC DISPERSER 17 15 ”HIGH SHEAR” ROTOR-STATOR MIXER 17 16 COAXIAL JET FLOW MIXER 18 17 SIDE ENTRY JET FLOW MIXER 18



18 KENICS STATIC MIXERS 19 19 ROSS LLPD MOTIONLESS MIXER 19 20 SULZER SMV 20 21 SULZER SMX 20 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 21



0 INTRODUCTION/PURPOSE This Guide is one in a series of Mixing Guides and has been prepared for GBH Enterprises. 1 SCOPE This Guide directs the user to the most appropriate Guides to solve a mixing problem. It does so by classifying the mixing systems, by asking a series of questions and by defining the common terminology. 2 FIELD OF APPLICATION This Guide applies to Process Engineers in GBH Enterprises worldwide. 3 DEFINITIONS No specific definitions apply to this Guide. 4 THE MIXING GUIDES 4.1 Mixing Guides Guidance on Mixing is available from two sources, the Mixing Guides and the GBHE Mixing and Agitation Manual. Mixing Guides are intended to provide initial guidance and a quick design for uncomplicated mixing tasks in research, development and production. They concentrate on recommendations and design or scale-up procedures. The Mixing Guides are designated GBHE-PEG-MIX-XXX where the last three digits identify the individual guides.



4.2 GBHE Mixing and Agitation Manual The GBHE Mixing and Agitation Manual contains more detail, background to the correlations and recommendations, many more references and is the repository of the collective know-how. It provides greater insight and is thus better placed to advise on more complex or unusual mixing problems. 5 DEVICE SELECTION The range of devices covered by this series of Guides is summarized in Table 1. Many of the duties could be performed by two or three different devices; it is worthwhile considering the options before embarking on the detail design. Appendix A illustrates typical rotating mixing devices and some typical ratios of dimensions for agitated tanks. Appendix B shows devices without moving parts, i.e. static and coaxial jet mixers.



TABLE 1 SUMMARY OF TYPICAL USES OF MIXING DEVICES



6 MIXING QUESTIONNAIRE The objective of the following questions is to act as a check list for the designer. The list is not, however, exhaustive but is intended to stimulate ideas. 6.1 What is being mixed? Type of Mixture Mixing Guide GBHE Mixing and

Agitation Manual

Miscible Liquids GBHE-PEG-MIX-701 Section A Gas-Gas GBHE-PEG-MIX-702 Section B Solid-Liquid GBHE-PEG-MIX-703 Section C

GBHE-PEG-MIX-709 Section D4

Immiscible Liquids GBHE-PEG-MIX-704 Section D GBHE-PEG-MIX-709 Section D4

Gas-Liquid GBHE-PEG-MIX-705 Section E Gas-Solid-Liquid GBHE-PEG-MIX-706 Section F Solid-Solid GBHE-PEG-MIX-707 Section G Gas-Solid GBHE-PEG-MIX-708 Section H 6.2 Why is it being mixed? Because a mixture is required e.g. in paint manufacture

How well does it need to be mixed? To promote mass transfer - To react is the product spectrum dependent

on the rate of mixing?



Mass transfer followed by Which process is rate limiting? Reaction Is it important which is limiting? 6.3 How is it to be Mixed? Batchwise in an - Agitated Tank

Jet Stirred Tank Flow Mixer in Loop

Continuously in an - Agitated Tank

Flow Mixer Bubble Column

'Semi-batch’ in a - Bubble Column

Two-Phase Agitated Tank 6.4 Is Heat Transfer Important? Provision of heat transfer surface could determine the size of the equipment. In-line (static) mixers could be incorporated within a shell and tube heat exchanger. 6.5 Is Mixing Time Important? Reactions are often potentially very rapid and in practical equipment their rates can be determined by the rate of mixing of the reactants. 6.6 Is Inventory Important? Nitroglycerine is made in jet mixers to minimize the inventory of unstable material. Similar considerations could apply to other hazardous (including toxic) materials. 6.7 Is Subsequent Phase Separation Important? Mixing intensity (and hence bubble or droplet size) may need to be optimized to take account of the ease of separation.



6.8 What Quantities? With small quantities simplicity of operation may be the deciding factor while for large quantities the capital cost may influence the type of equipment to be used. 6.9 What are the Selection Criteria? Is the choice to be made primarily on economic grounds or is the process requirements dominant? 6.10 What Data are required? Apart from those data already listed above, the following are likely to be important: Viscosity is the liquid Newtonian? Surface Tension or Interfacial Tension is the effect of surfactants (or surface active impurities) important? Density - Diffusivity - Particle size is size and size distribution important? Reaction Rate - Vapor Liquid Equilibria - Liquid-Liquid Equilibria -



7 BASICS Mixing of fluids is one of the most common operations in chemical processing, yet also one of the most misunderstood as far as the fundamental mechanisms are concerned. Design of equipment is based largely on empirical correlations tempered with experience. For gas-liquid and liquid-liquid contacting, only scale-up relationships exist, as a priori design relationships for real systems are not yet available even on an empirical basis. Some understanding and quantification of the basic mechanisms is emerging however (especially for turbulent mixing) and this is treated later in this Clause. The situation is further complicated by the other duties a mixer often has to perform (such as heat transfer, chemical reaction and interphase contacting) and the constraints which may prevail (such as energy consumption (not often important), fluid inventory and sealing (important for safety), corrosion, mechanical robustness (especially agitator shafts), fouling and blockage by solids (e.g. in flow mixers) and foaming). In the case of chemical reactions the flow pattern in the mixer (backmixed or plug flow, continuous or batch) is often important also. It is vital that all such factors as are relevant are taken account of in selection and design of a mixer. This will often involve several iterations of the design calculations before the limiting factor is identified. The relative performance of a device for these duties will depend on the device selected, so several types of device should be considered initially. Several types of device for each duty are covered in the subsections of the Guide. The designer should obviously aim for the most economical design and this will often imply maximizing the rate of the limiting factor, within the process constraints. Mixing of liquids takes place by four mechanisms (bulk movement, shear and elongation, turbulence and molecular diffusion). For example, when a liquid B is added to a tank containing liquid A, with which it is miscible, it does not disperse immediately and there is obvious inhomogeneity: all of B is sitting in one part of the tank. When the stirrer (or jet) is started the liquid begins to move and all of B tends to move round the tank together. But, because the velocities are not uniform, B begins to spread out and this spreading continues until B is distributed all over the tank so that if a large sample of liquid is removed the quantity of B it contains will be independent of where the sample is taken from. The tank is now probably well mixed, but this is not necessarily so. If the flow in the tank is laminar, the close inspection of a small volume will reveal that there are layers of liquid A separated by layers of liquid B.



Eventually, the layers of liquid are sheared down to molecular dimensions and mixing is complete. Shearing down to molecular dimensions is not as difficult as it sounds: the thickness of the layers has to be halved less than 30 times. In practice this amount of shearing is rarely necessary because molecular diffusion spreads the material out of the layers. If the flow in the vessel is turbulent local mixing is enhanced by eddies, which are regions of highly sheared fluid which move through the fluid bulk, decaying in size. They are postulated to decay until they reach a minimum size, the 'Kolmogorov length scale' (Ѵ3/ε)1/4. On length scales below this size, energy is dissipated by viscous forces only.

The motion on a scale smaller than 32 µm is effectively laminar: thus neutrally buoyant particles that are appreciably smaller, e.g. 5 µm, will behave as if in a stagnant liquid. Thus, if they are soluble, the dissolution rate can be calculated simply by using:

To summarize; the three processes which are present in mixing operations are:



7.1 Bulk Movement The velocity is not uniform so the fluid is spread out by velocity differences and Distributive Mixing occurs. The fluid is split into different streams, for example at the agitator in a stirred tank, where one stream goes up and one down, or by an in-line mixer. Because the travel times are different, the two streams do not recombine but subsequently mix with other volumes of fluid. Bulk movement is usually the rate determining process in turbulent mixing (blending) in tanks as is shown by the fact that mixing times are not significantly affected by changes in viscosity. It also contributes significantly to mixing in in-line mixers. However, it is not of much significance in pipe flow. In laminar flow, bulk movement is the only flow which occurs and mixing time is determined by a balance between laminar shear and molecular diffusion, the former being assisted by distributive mixing effects. 7.2 Shear and Elongation Shear and elongation arise from bulk movement and are the mechanisms responsible for spreading out large clumps of fluid. 7.3 Turbulent Diffusion This is usually the rate determining process in pipe flow, where it is often modeled by using an effective diffusion coefficient. It is also relevant to mixing in any turbulent system, especially to mixing in a localized zone of a vessel. 7.4 Molecular Diffusion This is the ultimate process in all mixing operations, but in turbulent systems clumps of liquid are usually ground down so rapidly by shear in the eddies that it is not usually important in determining bulk mixing time. In laminar systems molecular diffusion can be important. The rates of fast localized reactions can also be controlled by molecular diffusion. The foregoing discussion has assumed that the liquids being mixed are miscible and of similar viscosities (within a factor of 10 or so). In the case of immiscible liquids the rate limiting process is usually the breakup of droplets and mixing times are likely to be much longer than those when bulk flow alone is limiting. In



the case of liquids of differing viscosities the more viscous component often refuses to join in the bulk flow and mixing times are determined by mass transfer at the interface between the two liquids. 7.5 Mixing Mechanisms The requirements of a mixing operation are met by setting up the hydrodynamics appropriate to the situation, which will involve combinations of bulk flow important for blending, solid suspension, heat transfer) and shear (for breakup of bubbles, droplets or particle agglomerates and interphase mass transfer). Figure 1 illustrates broadly the achievable characteristics of various devices in terms of bulk flow. FIGURE 1 CHARACTERISTICS OF VARIOUS DEVICES IN TERMS OF BULK FLOW

For agitators in vessels of given geometry, comparative performance for producing flow may be judged by comparing FLOW NUMBERS:



and some idea of comparative performance for shear in turbulent agitation may be gained by comparing POWER NUMBERS:

Often (e.g. in agitated vessels) flow is required to transport fluid through a localized high shear region.



APPENDIX A ROTATING MIXING DEVICES A.1 Figure 2 shows a tank and agitator with the key dimensions annotated and

a Table of Typical Ratios of these dimensions. Note that the base may be either flat or a dished end.

FIGURE 2 TANK LAYOUT



A.2 Figures 3 to 6 illustrate a variety of different agitators which are generally

used for mixing low viscosity non-Newtonian liquids. FIGURE 3 PITCHED BLADE TURBINE



FIGURE 4 DISC TURBINE

FIGURE 5 PROPELLER



FIGURE 6 RETREAT CURVED BLADE TURBINE

A.3 The agitators shown in Figures 7 to 9 are used primarily in gas-liquid

systems. For the suspension of particulate solids in a liquid, a Marine Propeller (Figure 5) or a Pitched Blade Turbine (Figure 3) is normally recommended.



FIGURE 7 CONCAVE BLADE TURBINE

FIGURE 8 'GASFOIL' AGITATOR



FIGURE 9 'HYDROFOIL' AGITATOR

A.4 For high viscosity and/or non-Newtonian systems, the selection is often

from those illustrated in Figures 10 to 15.



FIGURE 10 HELICAL SCREW STIRRER WITH THREE BAFFLES AT 120°

FIGURE 11 HELICAL RIBBON STIRRER



FIGURE 12 ANCHOR AGITATOR



FIGURE 13 BENT ANCHOR

FIGURE 14 ”SAWTOOTH” DISC DISPERSER



FIGURE 15 ”HIGH SHEAR” ROTOR-STATOR MIXER



APPENDIX B MIXING DEVICES WITHOUT MOVING PARTS FIGURE 16 COAXIAL JET FLOW MIXER

FIGURE 17 SIDE ENTRY JET FLOW MIXER

FIGURE 18 KENICS STATIC MIXERS



FIGURE 19 ROSS LLPD MOTIONLESS MIXER

FIGURE 20 SULZER SMV

FIGURE 21 SULZER SMX



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: ENGINEERING GUIDES GBHE-PEG-MIX-701 Mixing of Miscible Liquids

(referred to in 6.1 and Table 1) GBHE-PEG-MIX-702 Gas Mixing

(referred to in 6.1 and Table 1) GBHE-PEG-MIX-703 Mixing of Solid-Liquid Systems

(referred to in 6.1 and Table 1) GBHE-PEG-MIX-704 Mixing of Immiscible Liquids

(referred to in 6.1 and Table 1) GBHE-PEG-MIX-705 Mixing of Gas Liquid Systems

(referred to in 6.1 and Table 1) GBHE-PEG-MIX-706 Gas-Solid-Liquid Mixing Systems

(referred to in 6.1 and Table 1) GBHE-PEG-MIX-707 Solids Mixing

(referred to in 6.1) GBHE-PEG-MIX-708 Gas Solid Mixing

(referred to in 6.1) GBHE-PEG-MIX-709 'High Shear' Mixers

(referred to in 6.1 and Table 1) OTHER GBHE DOCUMENTS GBHE Mixing and Agitation Manual (referred to in Clause 4 and 4.2).

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